Screw extruder with improved mixing pins

ABSTRACT

An extruder, in particular for elastomer material, is described, said extruder being of the type comprising an extruder body (1) provided with a cylindrical chamber (2) inside which an extrusion screw is rotatably inserted, and a plurality of mixing rods (4) inserted in the casing (1) of the cylindrical chamber (2) and projecting inside the said chamber (2) wherein each rod (4) is arranged tangential to the core of said screw(3) opposite interruptions in the thread of the screw (3) and passes through the whole of the chamber (2) engaging in both the opposite walls of said casing (1). &lt;IMAGE&gt;

The present invention relates to an extruder provided with mixing pins.

As is known, screw extruders are generally composed of an extruder bodywith a cylindrical chamber containing, longitudinally, a helical screwwhich, when rotated, pushes, mixes and compresses the extrusion materialtowards an extrusion head on which a die is mounted.

According to the prior art it is also envisaged, in certain cases, thatthe cylindrical chamber contains mixing pins or rods. As can be seenfrom FIGS. 1A and 1B, which show an extrusion cylinder according to theprior art, the pins are normally arranged at regular intervals in thelongitudinal direction and inserted radially in the casing of theextruder body.

In order to prevent interference during operation, the pins are fit inthe casing walls, opposite interruptions in the thread of the extrusionscrew, and also project inwardly the cylindrical chamber by a distancewhich is undoubtedly less than the transverse dimension of the gapbetween the core of the screw and the internal surface of the saidchamber.

The main functions of the pins is that of creating an obstacle to thefeeding movement of the extrusion material, which is thus disturbed andmixed more thoroughly, and of exchanging heat with extrusion material.

However, at least two types of problem are associated with thisconfiguration.

A first problem is linked with the impossibility of correctly regulatingthe temperature of these pins.

U.S. Pat. No. 5,324,108 discloses a pin cylinder extruder wherein somepins are provided tangentially in the chamber. Such pins have aninternal dead hole where thermal fluid is made to circulate back andfort. Such a kind of circulation path for the thermal fluid is notefficient and requires that the internal diameter of the pins besufficiently large to accomodate an exit tube.

Moreover, in the event of one or more of the rods breaking, thefragments which break off end up inside the cylindrical chamber,contaminating the extrusion material and, in the worst instance,damaging the extrusion screw or the actual cylindrical chamber.

U.S. Pat. No. 5,480,227 discloses a screw extruder provided with somediagonally inclined pins. The wall of the cylinder receiving the screwhas an anular recess. Pins are located very close to a bead of the screwand partially embedded in said cylinder wall. In this way, the extrudedmaterial may come into contact with a small portion of the lateralsurface of the pins, resulting in a poor thermal exchange. Moreover,extruded material is chennelled between the pins but not actually mixed.

The object of the present invention is to overcome the problemsencountered in the prior art by providing a new and original design ofextruder in which the temperature of the pins can be easily controlledeven with small diameters, the extrusion material may be intimatelymixed and thermally regulated by the pins and it is possible to preventdamage resulting from possible breakage of these pins.

These objects are achieved by means of a screw extruder as described inclaim 1.

Further characteristic features and advantages of the device accordingto the invention will emerge, however, more clearly from the followingdetailed description provided by way of example and illustrated in theaccompanying drawings, in which:

FIG. 1A is a longitudinally sectioned view through an extruder bodyaccording to the prior art;

FIG. 1B is a cross-sectional view taken along the line I-I of FIG. 1A;

FIG. 2 is a partial longitudinal section through an extruder bodyaccording to the invention; and

FIG. 3 is a cross-sectional view along the line III-II in FIG. 2.

An extruder comprises, in a manner known per se, an extruder body 1containing a cylindrical chamber 2 which longitudinally houses anextrusion screw 3.

The extrusion screw 3 is composed of a core N and a thread F which isspirally wound around the core N with a constant or variable pitchdepending on the circumstances.

Rotation of the screw, which is obtained by means of known actuatingmeans (not shown), causes feeding and compression of an extrusionmaterial (in particular rubber, elastomer material or the like)introduced into the cylindrical chamber via a suitable loading opening(not shown).

According to the invention, mixing pin members 4 which pass through thewhole of the thickness of the facing walls of the extruder body 1 andthe cylindrical chamber 2 are provided. The pin members 4 are insertedand fixed inside suitable insertion holes in the extruder body so as tobe tangential, and very close, to the core N of the extrusion screw 3.

Each pin member is composed of a hollow rod 4 a which has, fixed to itsends, locking bushes 4 b (for example engaged with the respective rod bymeans of a thread) by means of which the pin members are locked tightlyon the extruder body.

At the point where the chamber 2 is passed through by one of the rods 4,the thread F has an interruption Fi.

Preferably, the rods are arranged in pairs on either side of the core N.Moreover, the pairs of rods are distributed longitudinally, alternatingat 90° with respect to each other (FIG. 2).

The rods 4 a are tubular, namely they have at least one internal flowchannel through which a heat-regulating liquid (for example oil or hotwater or a cooling fluid, depending on the circumstances) can becirculated.

As shown schematically in FIG. 3, the fluid is circulated by introducingit at one end of the hollow rod 4 a and allowing it flow out at theopposite end by means of suitable delivery and return pipes (not shown).

Further, as can be well appreciated from FIG. 3, between the rods 4 aand the internal surface of the cylinder wall a substantial clearance 5is left. In other words, at least a portion of said rods is whollywetted by the extrusion material. This allows the extrusion material topass all around the pin members, so as to contact a major portionthereof and enhance the thermal exchange effect. Moreover, the twoextrusion material streams, coming from the two sides of the rod 4 aduring extrusion, meet each other downstream of the rod 4 a resulting inan enhanced mixing effect.

The rods according to the invention allow a continuous flow of aheat-regulating fluid, such that it is possible to control in an optimummanner the temperature in accordance with the specific requirements.Moreover, since they are fixed to the extruder body at both ends, evenif the portion inside the cylindrical chamber should break, theynevertheless would remain attached to the extruder body without theirfragments being able to end up inside the chamber.

This latter characteristic feature is of considerable importance andmeans that major savings may be made with regard to repair andmaintenance costs.

It is understood, however, that the scope of protection of the inventiondescribed above is not limited to the particular embodiment shown, butthat it is extended to any other constructional modification whichachieves the same usefulness.

For example, although mixing pin members with a circular cross-sectionhave been described, it is also possible to use, in particularcircumstances, tubular rods with a differently profiled cross-section.

What is claimed is:
 1. Extruder of the type comprising an extruder bodywith a cylindrical chamber inside which an extrusion screw is rotatablyinserted, a plurality of mixing thermally regulated pin members beinginserted through ceasing of said cylindrical chamber and being fixedwith their respective ends to the wall of said extruder, characterizedin that at least a portion of said rods projects inside said cylindricalchamber in such a way that its entire circumferential surface may bewetted by the extrusion material so as to enhance heat exchange andmixing effect.
 2. Extruder according to claim 1, in which said pinmembers comprise an hollow rod and a heat-regulating fluid is circulatedinside them.
 3. Extruder according to claim 2, in which said rodsconsist of a tubular rod, the ends of which emerge externally from saidextruder body and are fixed thereon by means of fixing bushes. 4.Extruder according to claim 3, in which said pin members are provided inpairs, one on one side and one on the opposite side of the extrusionscrew.
 5. Extruder according to claim 4, in which said pairs of pinmembers are arranged longitudinally, alternating at 90° with respect toeach other.
 6. Extruder according to claim 2, in which said pin membersare provided in pairs, one on one side and one on the opposite side ofthe extrusion screw.
 7. Extruder according to claim 6, in which saidpairs of pin members are arranged longitudinally, alternating at 90°with respect to each other.
 8. Extruder according to claim 1, in whichsaid pin members are provided in pairs, one on one side and one on theopposite side of the extrusion screw.
 9. Extruder according to claim 8,in which said pairs of pin members are arranged longitudinally,alternating at 90° with respect to each other.